Figure 5. Switching frequency programmability

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RFSW [kΩ]

To improve the line transient performance, keeping the PWM gain constant versus the input voltage, the input

voltage feed-forward is implemented by changing the slope of the saw-tooth ramp, according to the input voltage

change (Figure 6. Feed-forward a).

The slope of the sawtooth also changes if the oscillator frequency is programmed by the external resistor. In this

way a frequency feed-forward is implemented (Figure 6. Feed-forwardb) in order to keep the PWM modulator gain

constant versus the switching frequency.

On the SYNCH pin the synchronization signal is generated. This signal has a phase shift of 180° with respect to

the clock. This delay is useful when two devices are synchronized connecting the SYNCH pins together. When

SYNCH pins are connected, the device with a higher oscillator frequency works as master, so the slave device

switches at the frequency of the master but with a delay of half a period. This helps reducing the RMS current

flowing through the input capacitor. Up to five ALED6000s can be connected to the same SYNCH pin; however,

the clock phase-shift from master switching frequency to slave input clock is 180°.

The ALED6000 device can be synchronized to work at a higher frequency, in the range 250 kHz-1500 kHz,

providing an external clock signal on SYNCH pin. The synchronization changes the saw-tooth amplitude, also

affecting the PWM gain (Figure 6. Feed-forwardc). This change must be taken into account when the loop stability

is studied. In order to minimize the change of PWM gain, the free-running frequency should be set (with a resistor

on the FSW pin) only slightly lower than the external clock frequency.

This pre-adjusting of the slave IC switching frequency keeps the truncation of the ramp saw-tooth negligible.

In case two or more (up to five) ALED6000 SYNCH pins are tied together, the ALED6000 IC with higher

programmed switching frequency is typically the master device; however, the SYNCH circuit is also able to

synchronize with a slightly lower external frequency, so the frequency pre-adjustment with the same resistor on

the FSW pin, as suggested above, is required for a proper operation.

The SYNCH signal is provided as soon as EN is asserted high; however, if DIM is kept low for more than TDIMTO

timeout, the SYNCH signal is no more available until DIM re-assertion high.

ALED6000

Oscillator and synchronization

DS13018 - Rev 4

page 10/45